Add "--recreate" option

By default, we reuse the existing virtualenv. This speeds things up
considerably.
Handle nonexistent virtualenvs gracefully
Inject exists()
Merge pull request #8 from myint/recreate

Reuse existing virtualenvs
Update README (regarding #9)
Bump version
Handle numbers in YAML file gracefully

Previously, we would get:

    $ travis-solo
    'float' object has no attribute 'startswith'